The invention relates to a tuning arrangement comprising a tunable RF-circuit, a mixer stage coupled thereto, an oscillator circuit having a tunable resonant circuit and, for both circuits, a common tuning to mutually different resonant frequencies, of which at least the circuit having the higher resonant frequency comprises a parallel arrangement of, on the one hand, a circuit inductance and on the other hand a series arrangement of a tuning capacitor and a padding capacitor.
Such a tuning arrangement is described in "Philips Data-handbook--Components and Material", part 2, December 1982, pages 19-35, having the type designation ELC 2006.
In this known tuning arrangement, the two circuits are tuned with the aid of mutually equal, variable capacitance diodes to which the same tuning voltage is applied from the common tuning voltage input, the tuning frequency of the oscillator resonant circuit being higher than the tuning frequency of the RF-circuit, so that a variation in the tuning voltage results in a variation in the tuning frequency, which is greater for the oscillator resonant circuit than for the RF-circuit. This tuning-voltage dependent variation of the tuning frequency-alternatively referred to as the tuning slope--of the oscillator-resonant circuit is reduced by means of the padding capacitor. As a result thereof, the frequency-dependent deviation of the difference between the tuning frequencies of the two circuits is indeed reduced, but an appropriate parallel variation of these tuning frequencies is only achieved in a limited portion of the tuning range: particularly at lower tuning frequencies, said difference was found to decrease with frequency to an unwanted high degree. To prevent this, the padding capacitor can be switched to a lower capacitance value, for example, by switching a circuit capacitance in parallel with it.